Automatic gas purge system for ionization chamber used in radioactive sample analysis



D. H. FRANK Nov. 3, 1964 3,155,829 AUTOMATIC GAS PURGE SYSTEM FOR IoNIzATIoN CHAMBER USED IN RADIoAcTIvE SAMPLE ANALYSIS 2 Sheets-Sheet 1 Filed May 1'7, 1962 E BY I (n Il H. FRANK AUTOMATIC GAS PURGE SYSTEM FOR IONIZATION CHAMBER USED IN RADIOACTIVE SAMPLE ANALYSIS 2 Sheets-Sheet 2 INVENTOR.

Nov. 3, 1964 Filed May 17. 1962 United States Patent O AUTUMATIC GAS PURGE SYSTEM FOR lGNlZA- THEN CHAMBER USED M RADIACTVE SAM- PLE ANALYSES Donald H. Franti, (hieago, lll., assigner to Nuclear- Chicago Corporation, a corporation of Delaware Filed May 1.7, 1962, Ser. No. 195,533 7 Claims. (Cl. 25u-83.6)

This invention relates to an improved ionization gas chamber' device for radiation detection and measurement. In addition to my novel device, my invention also relates to an improved method of radiation detection.

The term ionization gas counting chamber is commonly applied to indicate a device wherein detection and measurement or counting of the disintegration rate of radioactive samples is carried out in a controlled gas atmosphere. The principle of operation is dependent upon ionization of a counting gas by emanations from radioactive material. ln the presence of a high voltage electrical iield, counting gas ions formed within the charnber produce a measurable and detectable electrical signal.

In many cases it is desirable to measure both the number of radioactive emanations from a sample as Well as the total relative energy of these radiations. Both the number of radiations and the energy of the radiations may be measured in a gas ionization chamber when the characteristics of the counting gas and the electrostatic iield within the chamber are suitably controlled.

lt is characteristic of certain counting7 gases that the ionization, that is the total ion charge, produced by a radioactive disintegration of a given energy is quantitatively reproducible. On the other hand, ionization of most gas mixtures will produce an uneven quantity of charged ions and therefore an irregular or variable electrical signal in a gas counting chamber. The term proportional counter is commonly applied to gas counting chambers wherein the ionization of the counting gas produces an electrical pulse which varies in intensity as a function of the energy loss of the ionizing particle. Two typical counting gases in common use in proportional counters are methane and a mixture of argon and methane. The term Geiger counter is commonly applied to gas counting chambers wherein the ionization of the counting gas produces an electrical pulse which is more or less constant in intensity notwithstanding wide variations in the energy of any specific ionizing radiation. Typical counting gases in common use in Geiger counters are helium isobutane mixtures, and argon or neon with halogen quench mixtures.

The gaseous atmosphere of the counting chamber is contaminated with nitrogen, oxygen, and other substances in the air whenever the chamber is opened, such as when samples which are to be measured are placed in the chamber. Before resuming the analytical counting process in the controlled gaseous atmosphere, a gas purge cycle by means of which contaminating gases are flushed from within the chamber must precede the counting process. In normal operation a gas ionization chamber is operated with a small low velocity ilow or counting how of the counting gas passing therethrough.

Data in the form of electrical pulse signals from the ionization gas chamber are introduced into suitable electronic analysis circuits which in turn derive from the characteristics of the electrical pulse signals the desired information which is then recorded or displayed. Electrical pulse signals obtained from a gas ionization chamber under conditions in which the gas is not suitably controlled 3,155,829' Patented Nov. 3, 1954 ice with respect to purity or chemical composition, pressure, or density and other characteristics will be spurious and yield unreliable or nonreproduci'ole results.

Accordingly, one object of my invention is to provide an improved apparatus for making highly reliable radioactive measurements in which a controlled gaseous atmosphere must be maintained.

Another object of my invention is to provide means for preventing spurious electrical pulse signals from a gas ionization chamber from passing into electronic analysis circuits where such spurious electrical pulse data would introduce information errors.

Another object of my invention is to provide a radiation gas chamber counting device having a gas chamber for holding radioactive material samples, the chamber being equipped with a simple error proof locking and opening means.

Still another object of my invention is to provide an improved gas counting chamber device in which the sequential operation of the gas purge cycle proceeds automatically without the use of additional manual adjustments.

Another object of my invention is to provide an improved nuclear radiation counting chamber which automatically interrupts propagation of pulse signal data to electronic processing circuitry when the chamber is open and automatically initiates the proper sequence'of steps necessary to prepare the chamber for counting, when the chamber is closed.

FGURE l is a side elevation View of a preferred embodiment of my invention.

FIGURE 2 is a top View of the embodiment of my invention shown in FIGURE 1.

FIGURE 3 is a fragmentary view of a horizontal cross section of the device shown in FIGURE l taken on line 3 3.

FIGURE 4 is a fragmentary view of a transverse cross section of the device shown in FIGURES l and 2 taken on line 4 4.

FIGURE 5 is a schematic Wiring diagram of the circuits used in combination with the device of FGURE l.

A preferred embodiment of the combination of mechanical and electrical components, which comprise my invention are illustrated in the accompanying drawings.

An ionization chamber liti comprised of a hemispherical half 12 is positioned generally vertically above a cylindrical half 14 of the chamber. A. chamber iloor lo comprises the lower extremity of the chamber it). Radiovactive samples i8 may be positioned centrally in the chamber 10 supported by the iioor 16.

The two halves l2 and, 14 of the ionization gas chamber l@ are sealed against leakage of gas into or out of the chamber by means of an O ring seal 2t) which when the two halves of the chamber are juxtaposed and held iirmly together effects a tight gas proof seal. The mechanical means by which the two halves or the ionization gas chamber are juxtaposed and pressed together to form a seal is described below.

Gas ports 22 and 24 in the hemispherical halt" l2 provide in ilow and exhaust respectively to the ionization gas chamber lli. These gas ports 22 and 2li are connected through solenoid operated Valves and check valves, not shown in the'drawings, respectively to a pressurized container of counting gas and to an exhaust line opening into the atmosphere.

lAn electrode 26 mounted through an electrically insulating bushing 28 is positioned centrally within the hemispherical half 12 of the ionization gas chamber.

suesse@ The hemispherical half 12 of the ionization gas chamber is mounted within a chassis 3? comprised of brackets 32 and 34 which hold the hemispherical half of the chamber stationary. The brackets 32 and 34 are formed to provide sliding ways and 4.2 upon which a base 44 in which the cylindrical halt i4 of the chamber itl rests. The base 44 in eliect comprises a sliding drawer and may be pulled from beneath the front panel 35 suiiiciently far to expose the cylindrical half of the chamber En without removing the base i4 from sliding ways and d2. Bearing brackets 46 and 41S suitable for mounting a rotatable rod are xed in axial alignment below t-e base An eccentrically mounted rod Si? is mounted longitudinally beneath the base, free to rotate within the bearing brackets, but positioned to engage the cylindrical half of the ionization gas chamber and urge it against the hemispherical half 14 by means of the eccentric or cam shaped central section 51 of rod 50.

In brief summary, the upper hemispherical half i?, of the ionization gas chamber is rigidly mounted to the chassis .'i) so that the connections with the counting gas supply and exhaust will be readily maintained. The lower cylindrical half lli of the ionization gas chamber rests within the base 44 so that upon engagement with the cam shaped portion of eccentrically mounted rod E@ the cylindrical half 14 may be urged into firm juxtaposition with the O ring 150 and the hemispherical halt l2 of the gas chamber. Upon rotating the rod 59 and relieving the pressure on the O ring the base le may be slid along the ways and facilitate access to the cylindrical half of the ionization chamber and sample iloor.

The eccentric rod Sti may be rotated by turning the hand knob 52. A radially mounted pin or latch 54, attached to the concentric rod 5ta in spaced relationship to the front bearing bracket 46 engages the tapered bearing block S6. By this action the base 4d is raised into position so that the cylindrical half 14 of the gas ionization chamber is forced against the O ring Ztl beneath the hemispherical half 12 and the ionization chamber liti is sealed. The tapered block 56 is so shaped that the pin Sii, when fully engaged with the block 56, is locked into position and may be released only with positive counter clockwise rotation of the shaft in the embodiment shown in FIGURE l of the drawings.

A microswitch 58 is mounted on the chassis 3d and positioned in space relationship with the rod Sil and the radially mounted pin 54 attached thereto. The microswitch 58 is actuated by the pin when the base is slid its full length into the chassis and the rod is rotated into its locked position, such as shown in FIGURE l. The microswitch 58 upon closing initiates a sequence of electricallyl controlled steps which are described below.

Referring now to FIGURE 5 which shows a schematic circuit diagram of the electrical circuit used in the preferred embodiment of my invention shown in FIGURES l through 4, switch S8 is a sensitive double pole double throw switch commonly called a microswitch, the poles or contacts of which are indicated at 66 and 68. When the ionization chamber lil is closed and locked, contact 66 of the switch 58 is open and contact 68 thereof is closed. Power from cycle 120 volt mains 70 and 72 is connected through lead 74 to a thermal delay relay switch 76. The thermal delay relay switch 76 is comprised of a resistive element 78, thermally responsive bimetal Si), and contact 82 which is normally in a closed position as shown in the drawings when the bimetal is cold. By this arrangement the resistive element 78 is energized when power from the mains and 72 is passed through the gas ow valve solenoid 84. However, activation of the circuit comprising the thermal delay relay switch and the gas ilow valve solenoid 84 is dependent upon a second circuit which will be referred to as a holding circuit and which will subsequently be further described in detail.

Solenoid operated switch 33, comprised of a solenoid 9i) and three contacts 92, 94 and 95, is connected so that solenoid 9i? is across the mains 7i) and 72 when contact 66 of switch 5S is closed. If the seal on the charnber is opened, even momentarily, switch 5S is necessarily opened, contact 66 is closed, and contact 63 is opened. Solenoid 99 is then energized and contacts 92, 94 and 96 move simultaneously to their open positions, respectively.

rfhe electrode 26 in the chamber is connected to lead tZ which in turn is connected to contact 94. Lead 104 connects to a gate in the pulse data analysis circuitry which prevents the passage of pulse data from tiowing into the .analysis circuit. Lead 166 connects to the analyzer circuit and, when energized, opens the gate which then permits passage of pulse data into the analyzer circuit.

When the gas chamber is locked in the sealed position the switch 58 is actuated, thus opening contact 66 and closing Contact 68. However, the solenoid 99 remains energized through the holding circuit which comprises lead 74, contact 82, lead 93, and the contact 92 of switch At this time it is important to note that for the proper operation of the holding circuit the following switching criteria be met. When switch 5S is actuated, contact 68 should close momentarily before contact 66 is opened. On the other hand, if there is a relatively short but iinitc time delay between the opening of contact 66 and the closing of contact 63, it is imperative that the solenoid 99 exhibit a delay period in excess of the dead time of switch 53, thus remaining energized through the switching transient period of switch Sti. Typical values for the switching times of switch 5d and solenoid 9i) are, respectively, two milliseconds and twenty milliseconds. The importance of the aforementioned switching criteria will become apparent in the subsequent description of the holding circuit.

Simultaneous with the interval between the completion of the switch cycle of microswitch 5S and the completion of the slower switch cycle of solenoid switch 33, the thermal time delay switch element 7S and the gas purge solenoid valve 84 are energized through the circuit comprising main 72, contact 68, lead 6, contact 96, lead 74, and a return to main 70. By this arrangement a purge ow rate of counting gas passing through the chamber is assured as soon as the ionization chamber seal is locked and, by the sequence of actions in the circuits beginning with the closing of contact 63 of switch 53, solenoid 84 is energized.

After a period of application of power through the resistive heating element 78, typically 30 seconds, the bimetal 89 responds and opens the switch contact 82, deenergizes the valve solenoid 84 and removes power from ead 98, which then causes solenoid 9G to be deencrgized. Upon solenoid 90 deenergizing, contacts 92, 94 and 96 then open and return to the positions shown in FGURE 5. The thermal time delay resistive element 7S is deenergized when Contact 96 returns to open position. The circuit is thus restored to the state shown in FIGURE 5, the ow of counting gas through the chamber is reduced to a trickle, which can be adjusted by a bypass around the solenoid valve, and the puse data evaluating circuits through lead N6 are activated.

Whenever the ionization gas chamber seal 2t) is disturbed, the microswitch 5S immediately reacts, closing contact 66 which energizes relay SS and in turn switches contact 94 to connect to lead 94 which stops the pulse counting and evaluating circuits. When the gas chamber is then sealed and locked, the purge cycle timing sequence begins automatically. Relay coil 90 remains energized by means of the aforementioned holding circuit comprising the main 70, lead 7d, thermal time delay switch contact 82, lead 98, relay contacts 92, the solenoid relay switch coil 90, and return to the main 72. Thus the relay contacts $2, 94, and 96 remain in the lower or energized position through the switching period of switch 5S and continue to remain in the lower position until the thermal time delay switch contacts 52 open after the predetermined time delay period. After each interruption of the gas seal, the pulse data evaluating circuit is opened immediately. Before counting is again initiated after the gas seal is closed, a gas purge procedure is automatically cycled, after which the pulse evaluating circuit is returned to the active counting condition.

i claim:

1. A device for detecting and measuring nuclear radiations and feeding the data thus gathered into analysis circuits comprising in combination an ionization gas chamber having an electrode, mechanical means for sealing and locking a radioactive sample within the chamber, a counting gas source connected through means to the chamber, a first electrical switch actuated by the locking means, a time delay relay switch connected in series with the fr'st switch, an electrical solenoid switching means in series with the time delay switch which activates the flow of counting gas at purge rate through the chamber upon closing the first switch, the electrical solenoid switching means mounted to stop the purge rate flow of counting gas through the chamber upon activation of the .time delay switch, and a second electrical switch mechanically connected with the first switch which closes when the first switch opens, the said second electrical switch being connected between lthe electrode and the analysis circuits, whereby a uniform gaseous atmosphere is maintained in the chamber during detection and measurement of radioactive samples positioned therein, and spurious data is not passed from the ionization chamber electrode to the analysis circuits.

2. A device for detecting and measuring nuclear radiation comprising an ionization gas chamber, the ionization gas chamber having an electrode mounted therein, mechanical means for opening and closing the chamber, a source of counting gas connected to the chamber, a first electrical switch actuated upon the mechanical means closing the chamber, and in combination an electrical circuit comprising a time delay switch in series with the first switch, a solenoid valve having an open position for rapid purge rate flow of gas therethrough when the solenoid is energized for controlling the flow of counting gas into the chamber the solenoid of the solenoid valve being connected to draw current through either the first electrical switch or the time delay switch such that counting gas flows into the chamber at purge rates during the period between closing of the first switch and the opening of the time delay switch, the gas flow rate being returned to normal flow rate after actuation of the time delay switch, and a switch for connecting the electrode in the chamber to lthe analysis circuitry by actuation of the first switch, whereby counting is initiated in the chamber' after the gaseous atmosphere thereof has been purged.

3. An apparatus for detecting and measuring nuclear radiation comprising a chassis, an ionization gas chamber mounted within the chassis having an upper hemispherical half and a lower cylindrical half, gas inlet and gas outlet ports in the chamber, an electrode positioned within the hemispherical half of the chamber, said electrode being connected to a signal output terminal, a source of counting gas connected through means to the gas inlet port, the gas outlet port connected through a valve means to exhaust, mechanical means for positioning and locking the cylindrical half of the chamber below the hemispherical half of the chamber, a first electrical switch mounted to be actuated by the locking means, a time delay switch connected in series to the first switch, an electrical switch for opening the valve means to the chamber inlet port actuated when the first electrical switch is closed, and a second electrical switch connected between the electrode and the signal output terminal mounted to close when the time delay switch closes, whereby the gas content 6 of the chamber may be uniformly controlled through a timed sequence determined by the sequential action of the rst switch and the time delay switch after locking the chamber.

4. A device for the detection and measurement of radiation from a sample of radioactive material comprising in combination a chassis; a gas iorization chamber mounted within the chassis; a gas inlet port in the chamber; a source of counting gas connected to the port through a gas flow control valve; a mechanical means for holding the sample within the chamber, the chamber operable between an open and loci e position and the mechanical means being slidably mounted within the chassis between said open and locked positions; locking means comprising an eccentrically mounted shaft extending beneath said mechanical means, said shaft having a pin extending radially therefrom; a means mounted on said chassis and adapted to receive said pin in a locking relationship; a first electrical switch actuated by the movement of said pin into said locked position; a time delay switch connected in series with the irst switch; electrical means for opening the gas iiow control valve into the chamber upon closing of the first electrical switch; and means for closing the gas flow valve when the time delay switch opens; whereby the gas contents of the chamber may be uniformly controlled after locking the sample in the chamber.

5. An apparatus for detecting and measuring nuclear radiation comprising a chassis, an ionization gas chamber mounted within the chassis having an upper hemispherical half and a lower cylindrical half, gas inlet and gas outlet ports in the chamber, an electrode positioned within the hemispherical half of the chamber, said electrode being connected to a signal output terminal, a solenoid valve means, a source of counting gas connected through the solenoid valve means to the gas inlet port, the gas outlet port connected through a check valve means to exhaust, mechanical means for positioning and locking the cylindrical half of the chamber below the hemispherical half of the chamber, the aforesaid mechanical means including an eccentrically mounted rotatable shaft mounted to urge upon rotation the lower cylindrical half of the chamber against the hemispherical half effecting a gas tight seal between the lower cylindrical portion of the chamber and the upper hemispherical portion of the chamber, a pin extending radially from the shaft, a means mounted on the chassis and adapted to receive the pin in locking relationship, a first electrical switch mounted on the chassis in spaced relationship with the shaft and positioned to be actuated by movement of the pin into or out of locked position, and electrical circuitry and means connected with the solenoid valve for controlling the rate of ilow of counting gas into the chamber, and for the connection of the electrode to the signal output terminal.

6. An apparatus for detecting and measuring nuclear radiation comprising the device of claim 5 in combination with electrical circuitry and means comprising a solenoid relay switch in series with the first electrical switch, the relay switch having a plurality of poles, a solenoid valve for controlling the counting gas iow rate, the solenoid valve being connected in series with the solenoid relay switch through a first pole thereof, a resistance thermal bimetal time delay switch having a vsingle pole, the thermal bimetal time delay switch resistor being connected in parallel to the valve solenoid and the normally closed time delay switch single pole connected in series with the valve solenoid, whereby when the first switch is closed the solenoid relay switch is activated, the valve solenoid is energized and a purge ow rate of counting gas through the solenoid valve and chamber is initiated, the resistor of the thermal bimetal time delay switch is energized and after a time interval opens the single pole, deenergizes the solenoid valve and returns the counting gas ow rate ing rate.

noid relay switch wherein a rst position connects the 5 chamber electrode to the signal output terminal, and the second position connects the electrode to the means for interrupting t ie connection, whereby when the first switch is closed and the counting gas is flowing at purge rates the electrode is connected through the second position to the signal interrupting means, and when the thermal time delay switch opens, the solenoid relay switch is deenergized and the third pole is returned to the first position and the chamber electrode s connected to the signal output terminal.

S References Cited by the Examiner UNITED STATES PATENTS 2,843,753 7/58 eeder Z50-106 2,917,634 12/59 Barnothy Z50-83.6 2,998,522 8/61 Martin 250-106 3,033,078 6/62 Kern Z50-106 3,085, 55 4/63 Kern 250-106 OTHER REFERENCES Automatic Sample Changer for Well Type Scintillaton Counters, International Journal of Applied Radiation and isotopes, vol. 4, December 1958, pages 118 to 121.

RALPH G. NLSON, Primary Examiner. 

1. A DEVICE FOR DETECTING AND MEASURING NUCLEAR RADIATIONS AND FEEDING THE DATA THUS GATHERED INTO ANALYSIS CIRCUITS COMPRISING IN COMBINATION AN IONIZATION GAS CHAMBER HAVING AN ELECTRODE, MECHANICAL MEANS FOR SEALING AND LOCKING A RADIOACTIVE SAMPLE WITHIN THE CHAMBER, A COUNTING GAS SOURCE CONNECTED THROUGH MEANS TO THE CHAMBER, A FIRST ELECTRICAL SWITCH ACTUATED BY THE LOCKING MEANS, A TIME DEALY RELAY SWITCH CONNECTED IN SERIES WITH THE FIRST SWITCH, AN ELECTRICAL SOLENOID SWITCHING MEANS IN SERIES WITH THE TIME DELAY SWITCH WHICH ACTIVATES THE FLOW OF COUNTING GAS AT PURGE RATE THROUGH THE CHAMBER UPON CLOSING THE FIRST SWITCH, THE ELECTRICAL SOLENOID SWITCHING MEANS MOUNTED TO STOP THE PURGE RATE FLOW OF COUNTING GAS THROUGH THE CHAMBER UPON ACTIVATION OF THE TIME DELAY SWITCH, AND A SECOND ELECTRICAL SWITCH MECHANICALLY CONNECTED WITH THE FIRST SWITCH WHICH CLOSES WHEN THE FIRST SWITCH OPENS, THE SAID SECOND ELECTRICAL SWITCH BEING CONNECTED BETWEEN THE ELECTRODE AND THE ANALYSIS CIRCUITS, WHEREBY A UNIFORM GASEOUS ATMOSPHERE IS MAINTAINED IN THE CHAMBER DURING DETECTION AND MEASUREMENT OF RADIOACTIVE SAMPLES POSITIONED THEREIN, AND SPURIOUS DATA IS NOT PASSED FROM THE IONIZATION CHAMBER ELECTRODE TO THE ANALYSIS CIRCUITS. 